Galactus is a good pick. Unfortunately, you'd have to introduce the Fantastic Four and they've had a notoriously hard time getting off the ground as a concept.
Also, after Thanos, upping the stakes will be impossible. It'd be a good time to focus on lesser villains for a bit or a premise that doesn't rely on a big bad. Maybe use the Skrulls and have an air of paranoia lingering around and focus more on character interactions than a singular, massive threat.
I love the idea of Dr. Doom. He is an amazing villain who really has not had a great on-screen adaptation yet. Also, why not Magneto? If they really are doing MCU X-Men films, then why not start out with the standard solo films and build to a team up of Magneto and other villains?
Eagle, Sixers, and Phillies Fan For Life. (Bigger fan than The Purge)